1. Field of the Invention
The present invention relates generally to the production of steam from silica-rich geothermal brine and especially to processes for using silicious sludge obtained from silica-rich geothermal brine as a by-product of such steam production.
2. Discussion of the Prior Art:
Large subterranean aquifers of naturally produced (geothermal) steam or hot aqueous liquids, specifically water or brine, are found throughout the world. These aquifers, which often have vast amounts of energy potential, are most commonly found where the earth's near-surface thermal gradient is abnormally high, as evidenced by unusually great volcanic, fumarole or geyser activity. Thus, as an example, geothermal aquifers are not uncommon along the rim of the Pacific Ocean, long known for its volcanic activity.
Geothermal steam or water has, in some regions of the world, been used for centuries for therapeutic treatment of physical infirmities and diseases. In other regions, such geothermal fluids have long been used to heat dwellings and in industrial processes. Although efforts to further develop geothermal resources for these site-restrictive uses continue, considerable recent research and development has been directed to the exploitation of geothermal resources for production of electrical power which can be conducted, often over existing power grids, for long distances from the geothermal sources. In particular, recent steep increases in the cost of petroleum products used for conventional production of electric power, as well as actual or threatened petroleum fuel shortages or embargoes, have intensified the interest in use of geothermal fluids as an alternative, and generally self-renewing, source of power plant "fuel."
General processes by which geothermal fluids can be used to generate electric power have been known for some time. As an example, geothermal steam, after removal of particulate matter and such polluting gases as hydrogen sulfide and ammonia, can be used in the manner of boiler-generated steam to operate steam turbine generators.
Naturally pressurized geothermal brine or water having a temperature of over about 400.degree. F. can be flashed to a reduced pressure to convert some of the brine or water to steam. The steam produced in this manner can then be used to drive steam turbine generators. The flashed geothermal liquid and the steam condensate obtained from power generation are typically reinjected into the ground to replenish the aquifer and prevent ground subsidence. Cooler geothermal brine or water can often be used to advantage in binary systems in which a low-boiling point, secondary liquid is vaporized by the hot geothermal liquid, the vapor produced being used to operate gas turbine generators; again, the cooled brine is typically reinjected into the ground.
As might be expected, use of geothermal steam is preferred over use of geothermal water or brine for generating electric power because the steam can be used more directly, easily and cheaply. Consequently, where readily and abundantly available, geothermal steam has been used for a number of years to generate commercially important amounts of electric power at favorable costs. For example, even by the late 1970's, geothermal steam at the Geysers in Northern California was generating about two percent of all the electricity used in California.
While energy production facilities at important geothermal steam sources, such as at The Geysers, are generally still being expanded, the known number of important geothermal steam aquifers is small compared to that of geothermal brine or water. Current estimates are, in fact, that good geothermal brine or water sources are about five times more prevalent than are good sources of geothermal steam. The potential for generating electric power is, therefore, much greater for geothermal brine and water than it is for geothermal steam. As a result, considerable current geothermal research is understandably directed towards the development of economical geothermal brine and water electric power generating plants, much of this effort within the United States being expended towards use of vast geothermal brine resources in the Imperial Valley of Southern California.
Although, as above-mentioned, general processes are known for using geothermal brine or water for production of electric power, serious problems, especially with the use of typically highly saline geothermal brine, are often encountered in practice. These problems are frequently so great as to prevent the production of electric power at competitive rates and, as a consequence, greatly impede the progress of flashed geothermal brine power plant development in many areas of the world.
These severe problems associated with the use of geothermal brines are principally caused by the usually complex composition of geothermal brine. At natural, aquifer temperatures in excess of about 400.degree. F. and pressures in the typical range of 400 to 500 psig, the brines leach large amounts of salts, minerals and elements from aquifer formations, the brines presumably being in chemical equilibrium with their formations. Thus, although brine composition may vary from aquifer to aquifer, wellhead brine typically contains very high levels of dissolved silica, as well as substantial levels of dissolved heavy metals such as lead, copper, zinc, iron and cadmium. In addition, many other impurities, particulate matter and dissolved gases are present in most geothermal brines.
As natural brine pressure and temperature are substantially reduced in power plant steam production (flashing) stages, chemical equilibrium of the brine is disturbed and saturation levels of impurities in the brine are typically exceeded. This causes impurities and silica to precipitate from the brine, as a tough scale, onto surrounding equipment walls and in reinjection wells, often at a rate of several inches in thickness per month. Assuming, as is common, that the brine is supersaturated with silica at the wellhead, in high temperature portions of the brine handling system, for example, in the high pressure brine flashing vessels, heavy metal sulfide and silicate scaling typically predominates. In lower temperature portions of the system, for example, in atmospheric flashing vessels, amorphous silica and hydrated ferric oxide scaling has been found to predominate. Scale, so formed, typically comprises iron-rich silicates, and is usually very difficult, costly and time consuming to remove from equipment. Because of the fast growing scale rates, extensive facility down time for descaling operations, unless scale reducing processes are used, is often necessary. Associated injection wells may also require frequent and extensive rework due to scale buildup and new injection wells may periodically have to be drilled at great cost.
Therefore, considerable effort has been, and is being, directed towards developing effective processes for eliminating, or at least very substantially reducing, silica scaling in flashed geothermal brine handling systems. One such scale reduction process, disclosed in U.S. Pat. No. 4,439,535 to Featherstone, et al, involves the induced precipitation of scale-forming materials, principally silica, from the brine in the flashing stage by contacting the flashed brine with silica or silca-rich seed crystals. When the amount of silica which can remain dissolved in the brine is exceeded by the brine being flashed to a reduced pressure, silica leaving solution in the brine deposits onto the seed crystals. Not only do the vast number of micron-sized seed crystals introduced into the flashing stage provide a very much large surface area than the exposed surfaces of the flashing vessels but also the silica from the brine tends to preferentially deposit onto the seed crystals for chemical reasons. Substantially all of the silica leaving the brine therefore precipitates onto the seed crystals instead of precipitating as scale onto vessel and equipment walls and in injection wells.
Typically, the crystallized silica precipitate is settled from the brine in a downstream reactor-clarifier stage, the clarified brine overflow therefrom being flowed on to a filtering stage and then to a reinjection stage. Some of the silica precipitate (sludge) from the reactor-clarifier may be pumped back upstream into the flash crystallization stage as seed material, the remainder being dewatered and removed from the facility for disposal. The amount of such silica sludge requiring disposal is, however, relatively large. For example, for a 10 megawatt power plant which requires a brine flow rate of about 1.3 million pounds an hour, more than six tons a day of silica sludge may be produced and require disposal.
During the silica crystallization process, many other materials precipitate from the brine onto the seed material along with the silica. The produced sludge, herein referred to as silica or siliceous sludge, although mostly silica, also typically contains significant amounts of barite and heavy metals, such as lead, copper and zinc, which, above specific levels of concentration, may be considered as toxic and may therefore require disposal at specially designated toxic waste dumps. The costs associated with disposal of toxic silica sludge are substantial and can be expected to increase as additional and larger geothermal brine power plants are constructed and more siliceous sludge is produced, as allowable concentrations of heavy metals in the sludge are reduced to meet anticipated stricter environmental requirements and as toxic waste dumps become fewer and/or more remotely located. Even when the siliceous sludge produced is non-toxic, it may, nevertheless, contain small particles and/or polluting materials which may, in time, be eroded or leached out by rain or ground water, thereby creating environmental problems unless care is taken to properly dispose of the sludge. Such sludge disposal, even when the sludge is non-toxic, can be very costly.
Siliceous sludge precipitated during the flashing of geothermal brine for the production of steam results in the precipitation of silica particles along with particles of heavy metal compounds. The precipitated particles are very small, typically having a mean particle size of between about 1 and about 17 microns depending upon the operating conditions of the flash vessels. This particle size is an inherent property of the particles of the siliceous sludge produced by flashing of geothermal brine.
Heretofore, fine particles, such as present in sludge precipitation from geothermal brine, have been considered deleterious substances, in concrete compositions, see Properties of Concrete, Neville, A. M., p. 152, Pittman Publishing, Inc., Mass., 1981. According to Neville, p. 152, a material with particle size between 2 and 60 microns is considered a silt or fine dust. Owing to its fineness, and therefore large surface area, silt and fine dust should be avoided because they increase the amount of water necessary to wet the particles in the mix. It is well known that the increased water content in the mix decreases the ultimate strength of the concrete. For example, the strength of concrete increases with increasing aggregate size, because the water to cement ratio can be lowered. This is set forth by Mindess, S. and Young, J. F., in Concrete, p. 124, Prentice Hall, N.J., 1981.
Therefore, because of the inherent particle size of geothermal sludge, it would be expected that a structural concrete could not be made therefrom. By way of reference, structural plain concrete is recognized by the American Concrete Institute as having a minimum strength of not less than 2500 psi. (The American Concrete Institute, ACI, Report No. 318.1-83, Chapter 4, paragraph 4.2).
Heretofore, most uses of sludge in concrete composition have been for the encapsulation of particles of toxic materials in order to prevent subsequent release of the toxic materials into the environment. For example, in U.S. Pat. No. 4,113,504 issued to Chen, et al, the heavy metal content of sludge waste is fixed in the cementitious solid product by mixing the waste sludge with fixing ingredients comprising vermiculite and cement. Examples of sludge wastes utilized in Chen, et al are mercury containing sludge from mercury cathode brine electrolysis cell operation and arsenic containing sludge from the combustion of elemental phosphorous to form phosphoric acid.
As pointed out in column 2, line 35 of Chen, essential ingredients necessary to fix the heavy metal content of the sludge are vermiculite and cement. The teachings of Chen, et al are for the encapsulation of sludge and not for the production of structural concrete. It is well known that vermiculite cannot be used in concrete where strength is required, see p. 133 of Concrete Technology, Vol. 1, Properties of Materials, Third Edition, Orchard, D. F., John Wiley and Sons, N.Y., 1973. It is stated therein that vermiculite is principally used as an aggregate for concrete use for thermal insulation, lightweight screens, lightweight blocks and for other purposes where strength is not required. Therefore, the use of vermiculite in a concrete composition necessarily results in a concrete having low strength.
The Chen reference does not set forth any specific strength data on the concrete product produced, no doubt, because it is not intended to be used as structural concrete. However, in examination of the examples of Chen, et al setting forth the properties of the resultant solid product, it is observed that the Chen, et al concrete product can be sampled by "digging into the mass with a spatula", see column 4, lines 43-45, column 5, lines 1-2. In column 5, lines 63-65 of Chen, et al, core samples of the hardened mixture are obtained by driving a tube into the mass. Clearly, the Chen, et al reference provides no teaching of the production of structural concrete by utilizing a sludge material since Chen's concrete product is so soft it can be sampled with a spatula. The object of Chen, et al's process is to encapsulate heavy metals, not produce structural concrete. It is specifically stated in Chen, et al at line 37, column 3, that: "It is not necessary that the solid product posses compressive strength or dimensional stability typical of cement for structural use."
One reference that teaches the use of a sludge in the production of structural concrete is U.S. Pat. No. 4,226,630 to Styron. This reference discloses that flyash may be added to provide for increased concrete strength.
While Styron discloses the formation of structural concrete (concrete having greater than 2500 psig compressive strength) large amounts of cementitious flyash must be used. For example, in column 4, lines 46-50, Styron stipulates that between about 10 wt. % and about 70 wt. % flyash based on the weight of the slurry (flyash plus sludge) is necessary to reach the desired composition.
On this basis, the dry weight of solids, in the sludge, to flyash is between 0.87 to 2.61 for the composition, according to Example 1 of Styron, between 0.66 to 1.98 according to Example 2, and between 0.93 and 2.79 according to Example 3, where the sludge was 29% solid by weight, 81% liquid, 22% solids by weight, 78% liquid, and 31% solids by weight, 69% liquid, respectively.
From a practical point of view, the amount of flyash necessary to form structural concrete according to Styron makes the production of such concrete economically unattractive.
The present invention is directed to a process for producing a structural concrete using sludge precipitation from geothermal brine at unexpectedly high ratios of dry weight ratios of siliceous sludge to cementitious material.
It is, therefore, an object of the present invention to provide a process for using geothermal sludge to make a novel concrete material which can be used for construction purposes, the cementitious material, used with the sludge to make the concrete, causing the small particles, impurities and contaminants in the sludge to be substantially fixed in the concrete.
Another object is to provide a novel composition of structural concrete using sludge from geothermal brine as a constituent thereof at very high dry weight ratios of sludge to cementitious material resulting in the economic production of structural concrete and simultaneous disposal of sludge from geothermal brine.
Additional objects, advantages, and features of the invention will become apparent to those skilled in the art from the following description.